Kinetics and mechanisms of the thermal decomposition of ethane - I. The uninhibited reaction

The uninhibited ethane decomposition was studied from 550 to 640°C with the object of determining the overall mechanism. The reaction was found to be accurately of the first order at the higher pressures and lower temperatures employed, and to have an activation energy of 73·1 kcal under these conditions. The rate was decreased slightly by an increase in surface area, and the order was then somewhat greater than unity. At 640°C there was a transition to an order of 3/2 at a pressure of about 60 mm. Evidence is adduced in support of the conclusion that the initiating reaction is a second-order split of C2H6 into 2CH3, as proposed by Küchler & Theile, and that the terminating step is C2H5 + C2H5 at the higher pressures and H + C2H5 at the lower ones. The mechanism is shown to give a satisfactory interpretation of the time-course of the reaction, of the effects of adding ethylene and hydrogen, and of the effect of increasing the surface area. Calculated rates, using the rate constants for the elementary steps, are in good agreement with experiment.